[rrdtool-maint] Bug#942639: fix build with Python3.8 + unusual packaging of the debug extension

[Matthias Klose]

Package: src:rrdtool
Version: 1.7.2-1
Severity: important
Tags: sid bullseye patch
User: debian-python at lists.debian.org
Usertags: python3.8

The attached patch fixes the build with Python3.8.

Noticing that you package both the normal and the debug extension module in the 
same package python3-rrdtool.  The latter should be packaged into a 
python-rrdtool-dbg package.